1. Field of the Invention
The present invention relates to a semiconductor light emitting device and more specifically relates to a semiconductor light emitting device capable of controlling optical output with high accuracy by monitoring the optical output.
2. Description of the Related Art
As light sources for optical communication, semiconductor light emitting devices having stable output characteristics within a wide temperature range have been required. In general, semiconductor lasers (laser diode LD) are used, and a LD is provided with an alignment mechanism for optical output so that optical output is little affected by temperature variations and other disturbances.
For example, a semiconductor light emitting device using the following alignment mechanism has been proposed. Front optical output of a LD is output to the optical fiber side, and rear optical output is received by a photodiode (PD) used as a light receiving element which functions as a monitor. In addition, a monitor current is generated according to the quantity of light received by the PD to adjust a LD driving current and control LD output (Japanese Unexamined Patent Application Publication No. 10-74972).
In another alignment mechanism, the epitaxial layer side surface of a PD used as a light receiving element is coated with a reflection film (HR film: High Reflection) having a predetermined reflection to apply optical output of a LD to the reflection film of the PD so that part of light is transmitted and output as monitor light to optical fibers, and most of light is reflected light and output as optical output to optical fibers (Japanese Unexamined Patent Application Publication No. 8-116127).
However, in the alignment mechanism using the rear optical output for monitoring, it is necessary to control the temperature dependence of the front optical output/rear optical output ratio, but control of the temperature dependence is difficult. Namely, when all the relations given below can be controlled, an alignment mechanism can be realized, in which the reflectance on the surface of a PD is changed according to a wavelength shift on the basis of the relation between the LD temperature and the wavelength shift.
(1) The relation between the LD temperature and the front optical output/rear optical output ratio.
(2) The relation between the LD temperature and the wavelength shift
(3) The relation between the wavelength and the reflectance of a reflection film
(4) The relation between the structure of a reflection film and reflectance
For example, in any case in which the wavelength of a LD is intentionally changed, the above relations cannot be controlled. Since it is necessary to strictly control all items including the film thickness of the reflection film, the production cost is increased. In brief, from a practical viewpoint, the complete control of the temperature dependence cannot be easily realized.
In addition, in the alignment mechanism in which part of light is transmitted and used for monitoring and most of light is reflected and used as output using the reflection film, a p-type portion electrode and wiring to an electrode pad are disposed near the center of the PD provided below the reflection film, and thus unevenness or a step shape of the wiring is reflected in the reflection film. Therefore, the reflectance and reflection direction of the reflection film are changed at a step portion, and thus output light is not output to the optical fiber side from this portion. As a result, even when light emitted from a LD has a normal intensity distribution (positional distribution of intensity), the intensity distribution of reflected light is greatly disturbed, thereby increasing noise and causing difficulty in using as an optical signal.
As a countermeasure for avoiding the disturbance in the intensity distribution of reflected light, a light receiving surface may be sufficiently widened for the spread of LD emitted light. However, this has the following problem: A p-type region is formed by Zn doping but has a limited doping amount and rate of electrical activation. And because of a relatively small thickness, when a current spreads in a transverse direction, the electric resistance is significantly increased. Thus, the driving voltage must be increased, and the frequency response characteristic is degraded due to the excessively high capacity. When the thickness of an epitaxial film of a p-type region is increased for decreasing the driving voltage, the cost of formation of the epitaxial film is increased. For example, when a fine mesh p-type electrode is provided for decreasing the electric resistance of a p-type region, a capacity due to the mesh electrode is excessively increased to degrade the frequency response characteristic. In addition, the fine unevenness of the mesh electrode inhibits normal reflection, thereby causing difficulty in using reflected light as an optical signal.